1. Field of the Invention
The present invention relates to a mechanism for adjusting the spacing or gap between the print head and platen of a serial printer in which characters and other images are printed on a print medium placed on the platen by the print head as it traverses the platen.
2. Description of the Related Art
Impact-type serial printers have a print head and a platen which are spaced from each other by a gap or spacing (hereinafter also referred to as a "platen gap") which is dependent on the response and output energy of the print head. The platen gap is required to be adjustable also dependent on the number of print media or sheets to be copied simultaneously and the thickness of a sheet or sheets placed between the platen and the print head.
Prior mechanisms for adjusting the platen gap are disclosed in Japanese Laid-Open Patent Publications Nos. 58-90975 and 58-175951, for example. The known platen gap adjusting mechanisms will be described with reference to FIGS. 6 and 7 of the accompanying drawings.
As shown in FIG. 6, a print head 1 is mounted on a carriage 2 with an eccentric bushing 3 fixed to an end thereof, the eccentric bushing 3 having an axial eccentric hole 4 defined therein. The carriage 2 has an elliptical hole 5 defined in the opposite end thereof remote from the eccentric bushing 3. Guide shafts 6, 7 are slidably inserted respectively in the hole 4 in the eccentric bushing 3 and the elliptical hole 5 in the carriage 2. A platen 8 is positioned such that it presents a print surface in confronting relation to the print head 1. The guide shafts 6, 7 and the platen 8 are supported in parallel relation by and between two frames 9, 10.
The carriage 2 with the print head 1 supported thereon is movable in the directions of the arrow 11 while sliding on and along the guide shafts 6, 7.
For adjusting the platen gap between the print head 1 and the platen 8, the eccentric bushing 3 is turned on the guide shaft 6 to displace the carriage 2 in the directions of the arrow 12. More specifically, as the eccentric bushing 3 is turned on the guide shaft 6, the eccentric bushing 3 is transversely displaced with respect to the guide shaft 6 to impart a similar transverse displacement to the carriage 2, which is allowed by the elliptical hole 5 in the carriage 2.
The platen gap is adjusted dependent on the number of print sheets to be copied simultaneously and the thickness of a sheet or sheets placed between the platen 8 and the print head 1.
Since each of the print head 1 and the carriage 2 is made up of a number of parts, the platen gap should be initialized taking into account the accumulated dimensional tolerances of the components of the print head 1 and the carriage 2. At the time the platen gap has been initialized, the angular phase of the eccentric bushing 3 varies from printer to printer. In addition, the eccentric bushing 3 can vary the platen gap to a limited extent since it is angularly movable for transverse displacement only in a limited range. For the above reasons, when varying the initialized platen gap to meet a print sheet or sheets used, a desired platen gap may not be achieved through the available angular movement of the eccentric bushing 3.
U.S. Pat. No. 4,390,292 to Krenz discloses a prior mechanism for adjusting the spacing between the print head and platen of a line printer.